International standards (particularly in America and Europe) relating to limiting emissions of polluting gases produced by motor vehicles, over the next few years have called for a progressive reduction in the emissions that can be released into the atmosphere (in particular a significant reduction of NOx and of particulate).
Compression ignition internal combustion engines (operating according to the Diesel cycle and using mainly diesel oil as fuel) have particular emission problems.
With appropriate technological upgrades, compression ignition internal combustion engines will be capable of satisfying, also in the future, the emission limitations established by these standards and, at the same time, also have excellent energy efficiency (over 40% in the most modern engines). With appropriate technological upgrades, positive ignition internal combustion engines (operating according to the Otto cycle and mainly using gasoline as fuel) will also be capable of satisfying, in the future, the emission limitations established by these standards; however, positive ignition internal combustion engines have lower energy efficiency (at most 33-34% in the most recent engines). In particular, the energy efficiency of positive ignition internal combustion engines is limited by the fact that, in order to avoid excessive detonation (which in the long term damages the cylinder and the piston), the compression ratio in the cylinders cannot be high.
Increased energy efficiency would lead the automobile market to increasingly use compression ignition internal combustion engines; however, existing refineries (which will continue to operate for many years) will be obliged to produce a certain quantity of gasoline during the oil refining process and therefore a substantial number (if not the majority) of automobile engines must continue to use gasoline as fuel. Consequently, great efforts are being concentrated on increasing the energy efficiency of positive ignition internal combustion engines that use gasoline as fuel.
In the past, to increase the compression ratio (and therefore the energy efficiency) while at the same time preventing excessive detonation, gasolines with additives (typically lead and/or manganese) having a higher octane number have been used; however, the use of these additives is no longer permitted by international standards and therefore other strategies must be found to improve the energy efficiency of positive ignition internal combustion engines.
As it is known, fuels are characterized by two indicators: cetane number and octane number, which are roughly inversely proportional.
Cetane number is an indicator of the behaviour of fuel during ignition; in other words, it expresses the readiness of the fuel to self-ignite, wherein the higher the cetane number is, the greater the readiness will be; instead, while the octane number expresses the anti-detonation property of the fuel. Diesel has a high reactivity (high cetane number and low octane number), while gasoline has a low reactivity (high octane number and low cetane number).
The document US20140251278 discloses the injection, into the cylinder of an internal combustion engine, of fractions of fuel that are appropriately heated through a specific active heater and subsequently injected through a single injector. In US20140251278 all the fractions are heated to the same temperature and substantially have the same reactivity. In particular, the injected mixture must preferably be in supercritical conditions. The main aim is to improve homogenization of the injection in supercritical conditions.
However, this solution has the disadvantage, above all for injections in high pressure environments (exceeding 50 bar) typical of internal combustion engines at the end of the compression stroke. The limitation of using a single injector and above all a single temperature leads to the risk of detonation if hot injection takes place too early (with the consequent need to reduce the compression ratio and therefore the efficiency of the engine) or the need to predominantly inject hot fuel close to the top dead centre, which leads to difficult control of the combustion and the pressure gradient during the first stages of combustion. Moreover, a large quantity injected close to the top dead centre leads to problems of particulate emissions, especially unless very high injection pressures (over 1000 bar) are used.
A positive or compression ignition internal combustion engine is also known to use gasoline as the predominant fraction of fuel (or low reactivity fuel) and a smaller fraction of diesel (or other high reactivity fuel); during the intake stroke gasoline is injected into the cylinder, while at the end of the compression stroke and close to the top dead centre of the piston, a small quantity of diesel or high reactivity fuel (i.e. high cetane number) is injected into the cylinder. In other words, this solution provides for fractionated injection in which two different fuels are injected at two different times.
A solution of this type is, for example, disclosed in the document EP2682588. With this solution, through the injection of a plurality of different fuels into the cylinder, a stratification of concentration and of reactivity is obtained, allowing improved control of combustion triggering, even in the absence of the ignition spark plug. This solution makes it possible to operate with very high compression ratios (typical of compression ignition internal combustion engines) while using a predominant quantity of gasoline as fuel, and without incurring excessive detonation before the injection of the fraction with a high cetane number. However, on the other hand, this solution is very costly and complex to produce, requiring doubling of the fuel supply system: in fact, a first supply system (injectors, pump and tank) is required for the gasoline and a second supply system (injectors, pump and tank) is required for the diesel (or other high reactivity fuel).